The present invention relates to a three-dimensional security feature (holographic fine-line pattern), and in particular, a three-dimensional security feature with enhanced counterfeit deterrent effect using hologram.
Conventionally, security features (fine-line patterns) created by printing have been used in the cash vouchers such as securities and banknotes for counterfeit deterrence. The conventional security features comprise geometric patterns with complicated combination of fine lines such as wave lines. However, due to the improvement of color copying machines in resolution and color reproducibility, counterfeit of these cash vouchers are increasing.
In addition, security features using diffraction gratings have also been practically used for imparting enhanced counterfeit deterrent effect. A security feature using the diffraction grating is often utilized as high level counterfeit deterrence measures as it can express the movement of light, and it can express fine patterns in high resolution, although in two-dimensional images. However, these security feature patterns using the diffraction grating which have been regarded as high level counterfeit deterrence technique are nowadays increasingly counterfeited, because the patterns recorded in it have come to be detected by microscopic observation and observation of the light movement, and because of the spread of the diffraction grating image forming devices using laser two-beam interference.
On the other hand, computer generated holograms (CGHs) have been known. There are generally two processes in the CGH producing technique, of which one is a process in which the object surface is replaced with a set of point light sources, known in the non-patent reference 1, 2, and others. The other is the use of the holographic stereogram, known in the patent reference 1 and the non-patent reference 3, and others.
The former process of the two, namely replacing the object surface with a set of point light sources, will be described here as a representative process.
As an example of CGHs, a binary hologram obtained by recording the intensity distribution of interference fringe, of which reconstructed image has parallax only in horizontal direction, and which is to be observed with white light from above, will be described in outline. Referring to FIG. 3, the shape of the object to be imaged in CGH is defined at step ST1. Then at step ST2, the space arrangements of the object, CGH plane, and reference light are defined. Then at step ST3, the object is divided in the vertical direction with horizontal slices, followed by replacement of the sliced surface with a set of point light sources. Then at step ST4, on the basis of these space arrangements, the intensities of the interference fringe between the light arriving from the point light sources constituting the object and the reference light are calculated for each sample point defined on the CGH plane, thereby obtaining the interference fringe data. Then at step ST5, the obtained interference fringe data are quantized. After that, at step ST6, the data are converted into a rectangular data for EB imaging, which are recorded at step ST7 on a medium by means of an EB imaging device, thus finally producing CGH.
In this calculation of the interference fringe, the hidden surface removal process is performed. The hidden surface removal process is a process of making a part, which is hidden by other object in front of it, invisible when an object is observed from a certain viewpoint, whereby the information of overlapping of objects is added to retina image, thus exhibiting a three dimensional effect. In the CGH recording, the hidden surface removal process is performed according to the following procedure.
As shown in FIG. 4, for each point light source constituting the object 1, the region in which the point light source is hidden by objects 1, 2 (the hatched area in FIG. 4) is obtained. In the case of CGH which is produced according to the procedure shown in FIG. 3, since the objects 1, 2 are sliced by horizontal surfaces and have parallax only in the horizontal direction, the region in which the point light sources on object 1 are hidden by objects 1, 2 is obtained from the positional relations between points and lines on each slice surface. The hidden surface removal process is a process in which, when a sample point of the interference fringe distributing on CGH plane is included in the region in which the point light sources are hidden obtained in the above (solid point in FIG. 4), that point light source at that sample point is eliminated from the calculation of the intensities of the interference fringe. From the image of object 1 reconstructed from CGH processed as above, the reconstruction light is not diffracted to the hatched area in FIG. 4, and the region of the object 1 corresponding to those point light sources becomes invisible because the region becomes behind the image of object 2 when an observer drops his viewpoint on that region.
In addition, it is also proposed in the patent reference 2 that color can be expressed with a CGH, produced by the process in which object surface is replaced by a set of point light sources, by reproducing the CGH with white light.
On the other hand, the inventor has proposed, in Japanese Patent Application No. 2001-365628, a CGH which is recorded in such a manner that a micro-object providing verifying information is arranged behind a covering object having a size easily recognizable by naked eyes, and the verifying information is hidden by the covering object and is not observable from a predetermined direction, but is observable from the other direction which is different from the predetermined direction. A representative example will be described with reference to FIG. 5. As shown in FIG. 5, verifying information which is a micro-object 11 such as a letter or a figure having such a size as not easily to be recognized by naked eyes, specifically having the largest size not greater than 300 μm, is arranged behind a covering object 12 having a size larger than micro-object 11 and easily recognizable with naked eyes arranged in front of the micro-object (nearer to the observer relative to the micro-object), at a position where micro-object 11 is covered by the covering object when viewed from the front, so that a viewer E can not observe the verifying information from the front (normal observing direction), and this arrangement is recorded in a CGH 10. For this end, the hidden surface removal process described above is performed on the set of point light sources expressing the micro-object 11, and the recording is performed in such a manner that the reconstruction light from the micro-object 11 does not diffract to a region at least between the line 21L and the line 21R in FIG. 5. The line 21L is a line passing the left end of the micro-object 11 and the left end of the covering object 12, and the line 21R is a line passing the right end of the micro-object 11 and the right end of the covering object 12, the front direction being included between the line 21L and the line 21R. In addition, the line 22L is a line drawn from the left end of the micro-object 11 toward upper left indicating a boundary of a region to which the reconstruction light from the left end of the micro-object 11 does not diffract, and the line 22R is a line drawn from the right end of the micro-object 11 toward upper right indicating a boundary of a region to which the reconstruction light from the right end of the micro-object 11 does not diffract.
In relation to the above, the right side emission angle γ2 of the object light of micro-object 11 is set larger than the angle β2 which is an angle between the line 21R connecting the right end of the micro-object 11 and the right end of the covering object 12 and the front direction, and the left side emission angle γ3 of the object light of micro-object 11 is set larger than the angle β3 which is an angle between the line 21L connecting the left end of the micro-object 11 and the left end of the covering object 12 and the front direction. Accordingly, as seen from FIG. 5, the angle range in which all or a part of the micro-object 11 is visible is γ2−β2+γ3−β3, while the angle range in which the micro-object is covered is β2+β3.
In this type of CGH, the presence of verifying information (micro-object 11) is difficult to be noticed because the recorded verifying information is too small to be recognized with naked eyes even under appropriate illumination. In addition, the presence of the verifying information is difficult to be noticed from the front direction which is the normal observation direction, even with the use of magnifying glass or other enlargement device, thus further enhancing the secrecy of verifying information and decreasing the danger of counterfeit.
In this type of CGH, the verification is performed by irradiating the hologram with appropriate illumination and observing it from a predetermined direction other than the front direction using magnifying glass or other enlargement device to reveal the verifying information (micro-object 11) The verifying information 11 can be confirmed as it disappears because it becomes behind the covering object 12 when the observation position is moved to the front where the observer's direction is the normal observation direction.
[Patent Reference 1]
Japanese patent No. 3,155,263
[Patent Reference 2]
Japanese unexamined patent publication 2000-214751
[Non-Patent Reference 1]
“Image Labo” April 1997 (Vol. 8, No. 4) p. 34-37
[Non-Patent Reference 2]
3D-Image Conference '99 Proceedings CD-ROM (Jun. 30-Jul. 1, 1999 at Kogakuin University Shinjuku Campus) “Image Type Binary CGH by EB Imaging (3)—The Enhancement of Three-Dimensional Effect by Hidden Surface removal/Shadowing”
[Non-Patent Reference 3]
Research Society of Holographic Display (Optical Society of Japan, Japan Society of Applied Physics), The Third Hodic Conference Proceedings (Nov. 15, 1995, at Nihon University Surugadai Campus, Building No. 1, Meeting Room No. 2) “The Speed Up of Two-Dimensional Image Sequence Generation for Holographic Stereogram”